It usually elicits many giggles and protests from the victim to stop.
Similar to a zerbit, this is done with your face is pressed in between the ass-cheeks of another person.
You then exhale out of your lips forcefully, similar to the way you would blow on a musical trumpet or do the "motorboat" to a pair of large breasts.
The resulting sound is similar to the sound of one passing gas.
I thought I was farting on his face when he gave me that farts nuggen!